This invention relates to a method of chemically cleaning sludge deposits from oil storage tanks by separating sludge into its hydrocarbon, aqueous and solid-matter components. Typically, sludge in a crude oil storage tank is composed of 40%-95% recoverable hydrocarbons. This invention is an economically and environmentally sound way to recover processable hydrocarbons as well as to remove sludge. Consequently, it minimizes wastes.
The present invention is an advancement over prior chemical cleaning methods which used oil based systems with dispersants. In said methods the sludge was typically sent to a crude unit. Sending the sludge to the crude unit passes high BS&W material through the desalters. As a result crude unit upsets often occur. The present invention does not send sludge components likely to cause crude unit upsets. Instead, via an aqueous chemical system, this invention sends only clean oil directly to the crude unit. The recovered hydrocarbon's value often exceeds tank cleaning costs.
Tanks are cleaned to repair leaks in the floor, steam coils, and roof drains. They are cleaned to facilitate removal or repair of sunken roofs. Tanks are also cleaned to recover storage capacity, and eliminate crude unit upsets. Crude unit upsets are caused by pluggage of the suction line to the crude charge pumps, slugs of water due to pluggage of the tank water draw, or slugs of solids because of high sludge levels in the tank. Additionally, cleaned tanks can be inspected and maintained to prevent their causing environmental harm.
Currently, federal environmental laws regulating waste disposal provide incentive for efficient chemical methods for cleaning storage tanks. Many standard tank cleaning methods, however, are too ineffective in minimizing waste to accord with federal laws. These methods include manual removal of sludge by workers inside the tank; mechanical removal by a front-end loader, which necessitates cutting a hole into the tank's side for entry; removal by remote control dredging devices; removal by power spray nozzles; and removal by circulating hot oil through the tank. Moreover, none of these methods in themselves recovers clean hydrocarbon; the sludge, after it is removed, must be taken elsewhere for hydrocarbon recovery.
Furthermore, hot oil methods and mechanical high shear methods form emulsions when water is circulated with the sludge. Emulsions would also likely occur if other standard surfactants, such as sulfonates, or sulfonate salts, were used. Emulsions make recovery of clean hydrocarbon more formidable. The present invention does not form emulsions.
In summary, the invention's main benefits are: (1) It recovers processable hydrocarbon, which is directly sent to the crude unit; (2) It minimizes waste by releasing water, which is routinely processable by the Waste Water Treatment Plant; and (3) It is emulsion free. Thus, the invention leaves only oil free solids to be landfarmed.
Other notable benefits include reduced tank downtime and reduced maintenance costs. By reducing crude tank downtime, the invention increases available crude storage capacity. This can have a major impact on refinery operations. By reducing manpower requirements, the invention generally lowers maintenance costs.